Among steel sheets used for beverage cans and food cans, steel sheets which are referred to as DR (Double Reduce) materials are used for lids, bottoms, three-piece can bodies, and drawn cans in some cases. The DR materials, which are cold-rolled (secondarily cold-rolled) again subsequently to annealing, are more readily reduced in thickness as compared with SR (Single Reduce) materials which are subjected only to a temper rolling with a small rolling reduction subsequently to annealing. The use of thin steel sheets allows can-making costs to be reduced.
The DR materials are work-hardened by cold rolling subsequent to annealing and therefore are thin hard steel sheets. However, the DR materials have low ductility and therefore are inferior in workability to the SR materials.
EOEs (Easy Open Ends) are widely used as lids for beverage cans and food cans.
In the course of manufacturing the EOEs, rivets for fixing tabs need to be formed by stretching and drawing. The ductility of a material that is required for such working corresponds to an elongation of about 10% as determined by a tensile test.
Body materials for three-piece cans are formed into a cylindrical shape and both ends thereof are then flanged for the purpose of swaging lids or bottoms. Therefore, end portions of can bodies also preferably have an elongation of about 10%.
On the other hand, steel sheets used as materials for making cans preferably have sufficient strength corresponding to the thickness thereof. In the case of the DR materials, which are thin, the DR materials preferably have a tensile strength of about 500 MPa or more for the purpose of ensuring the strength of cans.
It is difficult for the DR materials, which have been conventionally used, to achieve both the above ductility and strength. Therefore, the SR materials have been used for EOEs and body materials for beverage cans. However, in view of cost reduction, the use of the DR materials for such EOEs and body materials for beverage cans is increasingly preferred at present. Further, the materials can be used as materials for steel sheets for bodies of two-piece cans, DI (Drawn and Ironed) cans, DRD (Draw-Redraw) cans, aerosol cans, bottom ends, and the like.
In view of these circumstances, patent document 1 discloses a method for manufacturing a steel sheet having a high Lankford value and excellent flangeability by manufacturing a DR material from a low-carbon steel at a primary cold rolling reduction of 850 or less.
Patent document 2 discloses a method for manufacturing a DR material having a good balance between hardness and workability by treating low-carbon steel with nitrogen in an annealing step.
Patent document 3 discloses a method for manufacturing a lid for easy-open cans by scoring a thin steel sheet with a thickness of less than 0.21 mm such that the ratio of the residual score thickness to the thickness of the steel sheet is 0.4 or less, the steel sheet being obtained in such a manner that a steel slab containing 0.01% to 0.08% C, 0.05% to 0.50% Mn, and 0.01% to 0.15% Al is hot-rolled at a finish temperature not lower than the Ar3 transformation temperature, is then cold-rolled, is then recrystallization-annealed by continuous annealing, and is then skin-passed at a rolling reduction of 5% to 10%.
Patent document 4 discloses a continuously annealed DR steel sheet for welded cans and also discloses a method for manufacturing the same. The steel sheet has excellent flangeability equaling or exceeding that of batch-annealed DR steel sheets in case that the steel sheet contains 0.04% to 0.08% C, 0.03% or less Si, 0.05% to 0.50% Mn, 0.02% or less P, 0.02% or less S, 0.02% to 0.10% Al, and 0.008% to 0.015% N, the amount of (N total−N as AlN) in the steel sheet being 0.007% or more, and the steel sheet satisfies the relations X≧10% and Y≧−0.05X+1.4, where X is the value of total elongation of the steel sheet in the rolling direction thereof and Y is the value of average elongation thereof.    [Patent document 1] Japanese Unexamined Patent Application Publication No. 63-7336    [Patent document 2] Japanese Unexamined Patent Application Publication No. 2004-323905    [Patent document 3] Japanese Unexamined Patent Application Publication No. 62-96618    [Patent document 4] Japanese Unexamined Patent Application Publication No. 2007-177315